1. Field of the Invention
This invention relates to a display apparatus for vehicle to display speed of vehicle, warning and the like, generally called a head-up display (HUD).
2. Description of the Related Art
Japanese Utility Model Application Laid-Open No. Hei 7-42854 discloses such a display apparatus for vehicle. In this display apparatus as shown in FIG. 5, a projector 13 having a LCD display cell (not shown, hereafter referred to as a display device), a light source, a driving circuit, lens and the like is disposed in an instrument panel 1a of a vehicle 1. A displayed image on the display device of the projector 13 projected onto a windshield 2 is visible to eyes of a vehicle driver by reflecting the image with the windshield 2 toward the driver. A reflecting mirror 14 for compensating distortion which reflects the image of the display device onto an inner surface of the windshield 2 is provided in between the display device of the projector 13 and the windshield 2. This reflecting mirror 14 has reflection characteristics of the inner surface of the windshield 2. According to this structure, the reflected image of the display device distorted by reflection of the inner surface of the windshield 2 turns into an original normal image of the display device, so that the image on the windshield 2 can be very legible without distortion.
However, in the display apparatus for vehicle having above described structure, if a space for installing the projector 13 in the instrument panel 1a is small, there is a case where the reflecting mirror 14 cannot be installed in front of the projector 13.
Preferably, a distance between a displayed virtual image with lens and the driver is long. This is because there is merit that labor for focusing with eyes is reduced when the driver moves his eyes to the displayed virtual image during driving.
A distance between a reflecting point of a last reflecting mirror (windshield 2 in FIG. 5) and the display device corresponds to a distance between the reflecting point and the virtual image. Actually, because the windshield 2 is curved, the image displayed on the windshield 2 is enlarged and seems to be further.
Accordingly, it is necessary to physically dispose the display device away from the windshield 2 so that the distance between the display device and the windshield 2 satisfies the requirement. However, a space inside the instrument panel 1a does not have such a margin.
Therefore, there is a method making a virtual image by the display device and an optical member for magnifying the image such as a lens or a magnifying mirror in a display apparatus for vehicle having the small space inside the instrument panel 1a, whereby the display apparatus for vehicle has a same effect as a display apparatus for vehicle disposing the windshield 2 away from the display device.
Namely, as shown in FIG. 6, a relation between the display device and the optical member for magnifying is:1/a−1/b=1/f. 
A distance (b) between the optical member for magnifying (lens 5) and the virtual image 10 is determined by both a distance (a) between the lens 5 and the display device (display surface 3a of the LCD display cell ), and a focus f of a lens 5.
For miniaturizing the display apparatus, by reducing the focus f, the distance (d) is reduced to be able to miniaturize the display apparatus. However, such a display apparatus has a large display distortion and cannot provide a fine visible display.
Therefore, preferably, the display device and the optical member for magnifying are away from each other as far as possible in a given space, and a lens having the minimum magnification is employed as long as satisfying a requirement, so that the display apparatus provides a fine visible display with a minimum display distortion.
Moreover, for preventing the HUD unit from upsizing, there is a case where a certain amount of visible range is provided in an eye range. In this case, it is necessary for shifting the visible range throughout the whole eye range to adjust a position of the visible range in the eye range at the HUD unit side.